1. Field of the Invention
The invention relates to a process for preparing alkali metal alkoxides of higher alcohols from an alkali metal amalgam and the free alcohol.
2. Discussion of the Background
Alkali metal alkoxides are important intermediates for, inter alia, the pharmaceutical industry. They are also used as catalysts in the synthesis of many organic compounds. The alkoxides of sodium and potassium have achieved particular industrial importance. A number of methods are known for preparing alkali metal alkoxides (F. A. Dickes, Ber. Dtsch. Chem. Ges. 63, 2753 [1930]). Solutions of alkali metal hydroxides in an alcohol contain the corresponding alkali metal alkoxide in equilibrium. Removal of the water present in this equilibrium, e.g. by distillation, gives pure alkoxides. However, a large amount of energy is required for this method of shifting the equilibrium, particularly in the case of low-boiling alcohols.
Alkali metal alkoxides are obtained directly by "dissolving" an alkali metal in the corresponding alcohol. Here, sodium and potassium react violently with lower alcohols such as methanol and ethanol with evolution of hydrogen. The less reactive higher alcohols such as propanols and butanols are preferably reacted at above the melting point of the respective alkali metal, possibly under superatmospheric pressure while stirring.
However, alkali metals are expensive starting materials for the preparation of alkoxides. It is more economical to use the inexpensive, liquid alkali metal amalgams obtained in chloralkali electrolysis by the mercury process as alkali metal source. The use of catalysts for accelerating the reaction of alkali metal amalgam and alcohol is also known. Thus, the process described in EP-A-O 177 768 uses a bed of granular anthracite whose surface is coated with a heavy metal oxide or a mixture of heavy metal oxides. Alkali metal amalgam and alcohol are fed in continuously in a countercurrent manner and the alkali metal alkoxides are taken off continuously. A disadvantage of this process is that, in the preparation of alkali metal alkoxides of higher alcohols at acceptable reaction times, only from 60 to 80% of the alkali metal introduced in the form of the alkali metal amalgam can be reacted.
According to the proposal of German Patent Application 198 02 013.9, the alkali metal present in the alkali metal amalgam can be reacted to a greater extent even with higher alcohols at acceptable reaction times if the reaction is carried out in the presence of powder catalysts comprising transition metal carbides, nitrides or carbonitrides. Particularly suitable metals are molybdenum and tungsten and, of these, the carbides are particularly suitable. The powder catalysts are advantageously used at a mean particle diameter of from 1 to 10 .mu.m. The reaction is therefore referred to as a microheterogeneously catalyzed reaction.